How can I remove blank lines from a block of text with regex?

For text separated by LF (\n) or CRLF (\r\n), match a line made of zero or more spaces or tabs, followed by its line ending. The pattern is ^[ \t]*\r?\n: ^ marks the start of a line, [ \t]* accepts spaces or tabs (including none), and \r?\n matches LF with an optional preceding carriage return.

In Python, enable multiline mode with (?m) inside the pattern. In JavaScript, use the m flag so ^ works at line starts, and the g flag to replace every match. Python’s re.sub() and JavaScript’s replace() then substitute an empty string for those matches. [S1] [S2]

This handles lines containing spaces or tabs, not every character that a language might consider whitespace. It also does not match a whitespace-only final line if that line has no terminating newline; handle that case deliberately if your input can have one. [S1] [S2]

Assume the input uses LF newlines. Here, the second line contains spaces, and the tab before plum is part of a nonblank line:

Before (visible markers show line endings and whitespace): apple\n, ··\n, pear\n, →plum

After: apple\n, pear\n, →plum

The markers ·· and → are explanatory stand-ins for spaces and a tab; they are not literal characters in the input. The pattern consumes the blank line’s newline too, so it removes that line without deleting neighboring text. With CRLF input, the optional \r lets the same patterns match the carriage return before \n. [S1] [S2]

Here is the same assumed input and operation in each language:

Python:

import re

text = "apple\n  \npear\n\tplum"
cleaned = re.sub(r'(?m)^[ \t]*\r?\n', '', text)
print(cleaned)

JavaScript:

const text = "apple\n  \npear\n\tplum";
const cleaned = text.replace(/^[ \t]*\r?\n/gm, "");
console.log(cleaned);

Both produce apple\npear\n\tplum: the spaces-only line is removed, while the tab before plum stays because that line has text. Python’s (?m) activates multiline anchors; JavaScript’s m does that, and g makes replace() process repeated matches. These are language-specific flags and APIs, not interchangeable syntax. [S1] [S2]

First check the input’s actual line endings and what you mean by “blank.” These patterns target LF or CRLF lines containing only spaces or tabs. They do not establish behavior for every newline convention or every Unicode whitespace character. [S1] [S2]

Then check whether the final blank line has a newline after it. A trailing whitespace-only line without a line ending is a separate case; the shown patterns intentionally require one. If you want to remove that too, define the desired treatment of trailing spaces and a final empty line before changing the pattern. Test on a small copy and compare the result, especially if preserving the number of trailing newlines matters.